In producing a silicon single crystal, for example, by a pulling method, since a crucible for containing melt of a material is usually made of quartz, oxygen of about 10.times.10.sup.17 -20.times.10.sup.17 atoms/cc (old ASTM) is taken into the pulled crystal. During the fabrication of a device, the oxygen taken into the crystal is supersaturated and precipitated by heat treatment, thereby to create micro defects in the crystal. Such defects caused by the precipitation of oxygen can be a gettering site of impurities. Gettering which utilizes the defects caused by oxygen precipitation is called intrinsic gettering (hereinafter referred to as IG) which is widely used as one of clean gettering methods. In of clean gettering methods. In order to fully exhibit the advantageous effect of IG, precise control of the amount of the oxygen precipitation is required. In actuality, however, the amount of oxygen precipitation by heat treatment usually vary according to the thermal history of the crystal in the course of the pulling process.
The behavior of such precipitation of oxygen in a silicon wafer is described, for example, in ASTM Task Force Committee Report "Testing for Oxygen Precipitation in Silicon Wafers", Solid State Technology/March 1987, p. 85.
This report shows the conditions of heat treatment for silicon wafers having various initial interstitial oxygen concentrations (hereafter referred to as initial Oi) such as 1,050.degree. C. for 16 hours, or 750.degree. C. for 4 hours and then 1,050.degree. C. for 16 hours (see Table 1 for the conditions). In the report, heat treatment was performed in accordance with the heat treatment conditions of Table 1, and the difference of interstitial oxygen concentrations (hereinafter referred to as delta Oi) before the heat treatment (initial Oi) and after the heat treatment was measured. The result of the measurement is shown in FIGS. 6 and 7.
TABLE 1 ______________________________________ 1 1,050.degree. C. for 16 hrs 2 750.degree. C. for 4 hrs + 1,050.degree. C. for 16 hrs 3 800.degree. C. for 4 hrs + 1,000.degree. C. for 16 hrs 4 900.degree. C. for 4 hrs + 1,000.degree. C. for 16 hrs 5 1,000.degree. C. for 16 hrs ______________________________________
The heat treatment shown in Table 1 has been conventionally used as simple heat treatment conditions (hereinafter referred to as simulation heat treatment) for examining the amount of oxygen precipitation. FIG. 8 shows the yield of devices with respect to delta Oi as a result the simulation heat treatment. To ensure that the delta Oi falls within a hatched portion of FIG. 8, that is, in a range where the yield of device is not adversely effected, a plurality of wafers having substantially the same initial Oi is subjected to IG heat treatment at 650.degree. C. for 60 minutes as is performed conventionally. However, the delta Oi varies from wafer to wafer, and cannot always fall within the range. In the average, the delta Oi draws a S-curve with respect to the initial Oi, but the variance is large.
It is therefore an object of the present invention to reduce variance of the delta Oi in the device manufacturing process into a desired range of the delta Oi with high controllability so as to improve the yield of the devices.